A mix of straight lines, arcs, an ellipse (split) probably covers it. Join, trim and fillet to assemble, tidy and polish. If you are not yet comfortably familiar with how to do this then go here: Rhino - Learn to use Rhino (rhino3d.com) and start out with the Level 1 Manual - that will give you a more balanced foundation than most YouTube tutorials.
It isn’t clear how you want to use curvature: maybe in the plane of the rail, or perpendicular to it?
Or maybe you mean changing the cross-section to be more rounded at the arrows? - just add more cross-sections in different shapes and the sweep will try to accommodate them.
If you click on your icon on the forum page you can navigate to your preferences and add a name or nickname which better reflects who you are. There are now thousands of people with usernnnn user names which just seems very impersonal.